Deposition of photovoltaic thin films by plasma spray deposition

ABSTRACT

In particular embodiments, a method is described for depositing thin films, such as those used in forming a photovoltaic cell or device. In a particular embodiment, the method includes providing a substrate suitable for use in a photovoltaic device and plasma spraying one or more layers over the substrate, the grain size of the grains in each of the one or more layers being at least approximately two times greater than the thickness of the respective layer.

RELATED APPLICATIONS

This application claims the benefit, under 35 U.S.C. §119(e), of U.S.Provisional Patent Application No. 61/158,654, entitled DEPOSITION OFPHOTOVOLTAIC THIN FILMS BY PLASMA SPRAY DEPOSITION, filed 9 Mar. 2009,and hereby incorporated by reference herein.

TECHNICAL FIELD

The present disclosure generally relates to thin film deposition, andmore particularly, to depositing thin films using plasma sprayingtechniques.

BACKGROUND

P-n junction based photovoltaic cells are commonly used as solar cells.Generally, p-n junction based photovoltaic cells include a layer of ann-type semiconductor in direct contact with a layer of a p-typesemiconductor. By way of background, when a p-type semiconductor ispositioned in intimate contact with an n-type semiconductor a diffusionof electrons occurs from the region of high electron concentration (then-type side of the junction) into the region of low electronconcentration (the p-type side of the junction). However, the diffusionof charge carriers (electrons) does not happen indefinitely, as anopposing electric field is created by this charge imbalance. Theelectric field established across the p-n junction induces a separationof charge carriers that are created as result of photon absorption.

Chalcogenide (both single and mixed) semiconductors have optical bandgaps well within the terrestrial solar spectrum, and hence, can be usedas photon absorbers in thin film based photovoltaic cells, such as solarcells, to generate electron-hole pairs and convert light energy tousable electrical energy. More specifically, semiconducting chalcogenidefilms are typically used as the absorber layers in such devices. Achalcogenide is a chemical compound consisting of at least one chalcogenion (group 16 (VIA) elements in the periodic table, e.g., sulfur (S),selenium (Se), and tellurium (Te)) and at least one more electropositiveelement. As those of skill in the art will appreciate, references tochalcogenides are generally made in reference to sulfides, selenides,and tellurides. Thin film based solar cell devices may utilize thesechalcogenide semiconductor materials as the absorber layer(s) as is or,alternately, in the form of an alloy with other elements or evencompounds such as oxides, nitrides and carbides, among others.

Numerous deposition methods have been proposed for the preparation ofthe photovoltaic absorber layers, among other thin films, and theirprecursors. By way of example, such photovoltaic absorbers include CdTe,CuInS₂, Cu(InGa)Se₂, and CuInSe₂, among others, while their precursorsinclude, by way of example, CuIn, CuGaIn, and In₂S₃/Cu, among others.Conventionally, in the case of precursors, the constituent metalliccomponents are first deposited onto a substrate and then the finalcompound, a chalcogenide having chalcopyrite phase, is obtained throughthermo-chemical processing.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIGS. 1A-1D each illustrate a diagrammatic cross-sectional side view ofan example solar cell configuration.

FIG. 2 illustrates and example Plasma Spray Deposition system.

DESCRIPTION OF EXAMPLE EMBODIMENTS

The present disclosure is now described in detail with reference to afew particular embodiments thereof as illustrated in the accompanyingdrawings. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentdisclosure. It is apparent, however, to one skilled in the art, thatparticular embodiments of the present disclosure may be practicedwithout some or all of these specific details. In other instances, wellknown process steps and/or structures have not been described in detailin order to not unnecessarily obscure the present disclosure. Inaddition, while the disclosure is described in conjunction with theparticular embodiments, it should be understood that this description isnot intended to limit the disclosure to the described embodiments. Tothe contrary, the description is intended to cover alternatives,modifications, and equivalents as may be included within the spirit andscope of the disclosure as defined by the appended claims.

Particular embodiments of the present disclosure relate to the use ofPlasma Spray Deposition techniques in forming absorber structures foruse in photovoltaic devices (hereinafter also referred to as“photovoltaic cells,” “solar cells,” or “solar device”). In particularembodiments, Plasma Spray Deposition is used in forming chalcogenideabsorber layer structures. In particular embodiments, the disclosedtechniques may result in chalcogenide absorber layer structures in whicha majority of the materials forming the respective structures havechalcopyrite phase. In even more particular embodiments, greater than 90percent of the resultant chalcogenide absorber layer structures are inthe chalcopyrite phase after deposition by Plasma Spray Deposition, andin some embodiments, after subsequent thermo-processing such asannealing.

Hereinafter, reference to a layer may encompass a film, and vice versa,where appropriate. Additionally, reference to a layer may encompass amultilayer structure including one or more layers, where appropriate. Assuch, reference to an absorber may be made with reference to one or moreabsorber layers that collectively are referred to hereinafter asabsorber, absorber layer, absorber structure, or absorber layerstructure.

FIG. 1A illustrates an example solar cell 100 that includes, inoverlying sequence, a transparent glass substrate 102, a transparentconductive layer 104, a conversion layer 106, a transparent conductivelayer 108, and a protective transparent layer 110. In this example solarcell design, light can enter the solar cell 100 from the top (throughthe protective transparent layer 110) or from the bottom (through thetransparent substrate 102). FIG. 1B illustrates another example solarcell 120 that includes, in overlying sequence, a non-transparentsubstrate (e.g., a metal, plastic, ceramic, or other suitablenon-transparent substrate) 122, a conductive layer 124, a conversionlayer 126, a transparent conductive layer 128, and a protectivetransparent layer 130. In this example solar cell design, light canenter the solar cell 120 from the top (through the protectivetransparent layer 130). FIG. 1C illustrates another example solar cell140 that includes, in overlying sequence, a transparent substrate (e.g.,a glass, plastic, or other suitable transparent substrate) 142, aconductive layer 144, a conversion layer 146, a transparent conductivelayer 148, and a protective transparent layer 150. In this example solarcell design, light can enter the solar cell 140 from the top (throughprotective transparent layer 150). FIG. 1D illustrates yet anotherexample solar cell 160 that includes, in overlying sequence, atransparent substrate (e.g., a glass, plastic, or other suitabletransparent substrate) 162, a transparent conductive layer 164, aconversion layer 166, a conductive layer 168, and a protective layer170. In this example solar cell design, light can enter the solar cell160 from the bottom (through the transparent substrate 162).

In order to achieve charge separation (the separation of electron-holepairs) during operation of the resultant photovoltaic devices, each ofthe conversion layers 106, 126, 146, and 166 are comprised of at leastone n-type semiconductor material and at least one p-type semiconductormaterial. In particular embodiments, each of the conversion layers 106,126, 146, and 166 are comprised of at least one or more absorber layersand one or more buffer layers having opposite doping as the absorberlayers. By way of example, if the absorber layer is formed from a p-typesemiconductor, the buffer layer is formed from an n-type semiconductor.On the other hand, if the absorber layer is formed from an n-typesemiconductor, the buffer layer is formed from a p-type semiconductor.More particular embodiments of example conversion layers suitable foruse as one or more of conversion layers 106, 126, 146, or 166 will bedescribed later in the present disclosure.

In particular embodiments, each of the transparent conductive layers104, 108, 128, 148, or 164 is comprised of at least one oxide layer. Byway of example and not by way of limitation, the oxide layer forming thetransparent conductive layer may include one or more layers each formedof one or more of: titanium oxide (e.g., one or more of TiO, TiO2,Ti2O3, or Ti3O5), aluminum oxide (e.g., Al2O3), cobalt oxide (e.g., oneor more of CoO, Co2O3, or Co3O4), silicon oxide (e.g., SiO2), tin oxide(e.g., one or more of SnO or SnO2), zinc oxide (e.g., ZnO), molybdenumoxide (e.g., one or more of Mo, MoO2, or MoO3), tantalum oxide (e.g.,one or more of TaO, TaO2, or Ta2O5), tungsten oxide (e.g., one or moreof WO2 or WO3), indium oxide (e.g., one or more of InO or In2O3),magnesium oxide (e.g., MgO), bismuth oxide (e.g., Bi2O3), copper oxide(e.g., CuO), vanadium oxide (e.g., one or more of VO, VO2, V2O3, V2O5,or V3O5), chromium oxide (e.g., one or more of CrO2, CrO3, Cr2O3, orCr3O4), zirconium oxide (e.g., ZrO2), or yttrium oxide (e.g., Y2O3).Additionally, in various embodiments, the oxide layer may be doped withone or more of a variety of suitable elements or compounds. In oneparticular embodiment, each of the transparent conductive layers 104,108, 128, 148, or 164 may be comprised of ZnO doped with at least oneof: aluminum oxide, titanium oxide, zirconium oxide, vanadium oxide, ortin oxide. In another particular embodiment, each of the transparentconductive layers 104, 108, 128, 148, or 164 may be comprised of indiumoxide doped with at least one of: aluminum oxide, titanium oxide,zirconium oxide, vanadium oxide, or tin oxide. In another particularembodiment, each of the transparent conductive layers 104, 108, 128,148, or 164 may be a multi-layer structure comprised of at least a firstlayer formed from at least one of: zinc oxide, aluminum oxide, titaniumoxide, zirconium oxide, vanadium oxide, or tin oxide; and a second layercomprised of zinc oxide doped with at least one of: aluminum oxide,titanium oxide, zirconium oxide, vanadium oxide, or tin oxide. Inanother particular embodiment, each of the transparent conductive layers104, 108, 128, 148, or 164 may be a multi-layer structure comprised ofat least a first layer formed from at least one of: zinc oxide, aluminumoxide, titanium oxide, zirconium oxide, vanadium oxide, or tin oxide;and a second layer comprised of indium oxide doped with at least one of:aluminum oxide, titanium oxide, zirconium oxide, vanadium oxide, or tinoxide.

In particular embodiments, each of the conductive layers 124, 144, or168 is comprised of at least one metal or metallic layer. By way ofexample and not by way of limitation, each of conductive layers 124,144, or 168 may be formed of one or more layers each individually orcollectively containing at least one of: aluminum (Al), titanium (Ti),vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co),nickel (Ni), copper (Cu), zirconium (Zr), niobium (Nb), molybdenum (Mo),ruthenium (Ru), rhodium (Rh), palladium (Pd), platinum (Pt), silver(Ag), hafnium (Hf), tantalum (Ta), tungsten (W), rhenium (Re), iridium(Ir), or gold (Au). In one particular embodiment, each of conductivelayers 124, 144, or 168 may be formed of one or more layers eachindividually or collectively containing at least one of: Al, Ti, V, Cr,Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, Pt, Ag, Hf, Ta, W, Re, Ir,or Au; and at least one of: boron (B), carbon (C), nitrogen (N), lithium(Li), sodium (Na), silicon (Si), phosphorus (P), potassium (K), cesium(Cs), rubidium (Rb), sulfur (S), selenium (Se), tellurium (Te), mercury(Hg), lead (Pb), bismuth (Bi), tin (Sn), antimony (Sb), or germanium(Ge). In another particular embodiment, each of conductive layers 124,144, or 168 may be formed of a Mo-based layer that contains Mo and atleast one of: B, C, N, Na, Al, Si, P, S, K, Ti, V, Cr, Mn, Fe, Co, Ni,Cu, Zn, Ga, Ge, Se, Rb, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cs, Hf, Ta, W, Re,Ir, Pt, Au, Hg, Pb, or Bi. In another particular embodiment, each ofconductive layers 124, 144, or 168 may be formed of a multi-layerstructure comprised of an amorphous layer, a face-centered cubic (fcc)or hexagonal close-packed (hcp) interlayer, and a Mo-based layer. Insuch an embodiment, the amorphous layer may be comprised of at least oneof: CrTi, CoTa, CrTa, CoW, or glass; the fcc or hcp interlayer may becomprised of at least one of: Al, Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, Au, orPb; and the Mo-based layer may be comprised of at least one of Mo and atleast one of: B, C, N, Na, Al, Si, P, S, K, Ti, V, Cr, Mn, Fe, Co, Ni,Cu, Zn, Ga, Ge, Se, Rb, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cs, Hf, Ta, W, Re,Ir, Pt, Au, Hg, Pb, or Bi.

In particular embodiments, Plasma Spray Deposition may be used todeposit each of the conversion layers 106, 126, 146, or 166, includingone or both of the constituent absorber and buffer layers, which, asdescribed above, may themselves be comprised of multiple layers. Otherlayers described above may, in some embodiments, also be deposited withPlasma Spray Deposition, such as, for example, each of the transparentconductive layers 104, 108, 128, 148, or 164, as well as each of theconductive layers 124, 144, or 168. More particular embodiments ofabsorber layers suitable for use in, for example, conversion layers 106,126, 146, or 166, as well as methods of manufacturing the same, will nowbe described with reference to FIG. 2.

Copper indium gallium diselenide (e.g., Cu(In_(1-x)Ga_(x))Se₂, where xis less than or equal to approximately 0.7), copper indium galliumselenide sulfide (e.g., Cu(In_(1-x)Ga_(x))(Se_(1-y)S_(y))₂, where x isless than or equal to approximately 0.7 and where y is less than orequal to approximately 0.99), and copper indium gallium disulfide (e.g.,Cu(In_(1-x)Ga_(x))S₂, where x is less than or equal to approximately0.7), each of which is commonly referred to as a “CIGS” material orstructure, have been successfully used in the fabrication of thin filmabsorbers in photovoltaic cells largely due to their relatively largeabsorption coefficients. In fact, photovoltaic cells having photovoltaicefficiencies greater or equal than approximately 20% have beenmanufactured using copper indium gallium diselenide absorber layers.

Common or conventional deposition methods that have been proposed orused for depositing the constituent metallic components during thefabrication of photovoltaic absorbers, and particularlychalcogenide/chalcopyrite absorbers, include evaporation, sputterdeposition, electro-deposition, and printing/solution coating, amongothers. However, numerous problems and challenges are still faced withthese conventional deposition techniques. By way of example, rawmaterial variability is a major issue due to the relatively smallquantities that are used on a per batch basis (e.g., sputteringtargets). Additionally, system down time and service interruptionsconstitute a large fraction of operational time.

The goals of the deposition process include obtaining preferredchalcopyrite phases, defect free microstructures, stoichiometricconsistency, and thickness uniformity in the resultant thin filmabsorber layers. Additionally, in general, large grains are desirabledue to the minimization of grain boundaries and other imperfections thatcould serve as recombination centers for electrons in the semiconductingabsorber layer (or other layers) and thus degrade device performance.Low cost, broad process windows (e.g., larger tolerances and/or moreroom for error) and rapid manufacturing feasibility are also highlydesirable attributes as the photovoltaic industry enters the massmanufacturing phase.

Particular embodiments of the present disclosure utilize a techniqueknown as Plasma Spray Deposition (or plasma spraying) for fabricatingphotovoltaic devices, and in particular, the absorber layers of thesephotovoltaic devices. Additionally or alternately, the described PlasmaSpray Deposition techniques may be utilized to fabricate other thinfilms (e.g., buffer layers in the conversion layers, conductive layers,among others) in photovoltaic devices or other devices that include thinfilms. Plasma Spray Deposition is generally a method of thermal sprayingusing a plasma jet. Conventionally, Plasma Spray Deposition has beenused to deposit coatings having thicknesses from micrometers (μm) toseveral millimeters (mm) and, furthermore, has been used to depositcoatings from a variety of materials including, by way of example,metals, ceramics, polymers and composites. However, the presentinventors have determined that thin film absorber layer thicknesses asthin as approximately 1 nanometer (nm) may be precisely and uniformlyproduced using the Plasma Spray Deposition techniques described herein.

FIG. 2 illustrates an example Plasma Spray Deposition system 200. ThePlasma Spray Deposition process basically involves the spraying ofmolten or heat-softened material onto a surface to produce a coating,and more specifically as described herein, a thin film. In general, thematerial (or materials) to be deposited is referred to as feedstock(e.g., in the form of a powder, a liquid, a suspension, or wire) and isintroduced (injected) from a feedstock injection system 202 into theplasma jet 204, emanating from a plasma flame (torch). In the plasma jet204, where the temperature may be on the order of approximately 10,000Kelvin, the feedstock material is rapidly heated and melted and thenaccelerated to a high velocity and propelled towards a substrate 206 inthe form of molten droplets 208. The sprayed or ejected hot moltendroplets impact the substrate surface (or previously deposited layersthereon) and rapidly cool forming a thin film deposit. In particularembodiments, such a Plasma Spray Deposition process may be referred toas a “cold process” (relative to the substrate material being coated) asthe substrate temperature can be kept low during processing therebyavoiding damage, metallurgical changes, and distortion to the substratematerial.

In an example embodiment, the plasma sprayer (or plasma spray gun) 201includes a copper anode 210 and a tungsten cathode 212, both of whichmay be water cooled. Plasma gas (e.g, argon, nitrogen, hydrogen, helium,or other suitable gas) 214 flows around the cathode 212 and through theanode 210 which is shaped as a constricting nozzle. The plasma itselfmay be initiated by a high voltage discharge which causes localizedionization and a conductive path for a DC (direct current) arc to formbetween the cathode 212 and anode 210. The resistance heating from thearc causes the plasma gas 214 to reach extreme temperatures, dissociateand ionize to form a plasma (plasma flame) 216. The plasma 216 exits thenozzle formed by the anode 210 as a free or neutral plasma jet 204(plasma which does not carry electric current), which is quite differentto the Plasma Transferred Arc coating process where the arc extends tothe surface to be coated. When the plasma 216 is stabilized and readyfor spraying, the electric arc extends down the nozzle, instead ofshorting out to the nearest edge of the anode 210. This stretching ofthe arc is due to a thermal pinch effect. Cold gas around the surface ofthe water-cooled anode 210 nozzle, being electrically non-conductive,constricts the plasma arc, raising its temperature and velocity. Inembodiments using feedstock powder, the feedstock powder is fed into theplasma jet 204 via the feedstock injection system 202, which may includean external powder port mounted near the anode 210 nozzle exit.

The powder (or other feedstock) is so rapidly heated and accelerated bythe plasma jet 204 that spray distances can be in the order of 25 to 150mm. In some embodiments, the Plasma Spray Deposition process isconducted in normal atmospheric conditions (referred to as APS). Inalternate embodiments, the Plasma Spray Deposition process is conductedin protective or other suitable or desired environments using vacuumchambers normally back filled with a protective or other suitable ordesired gas at low pressure (referred to as VPS or LPPS).

Upon striking the substrate 206 (or previously deposited layersthereon), the molten droplets 208 flatten, rapidly solidify, and form adeposit. The deposited material remains adherent to the substrate 206 asa thin film 218. There are generally a large number of technologicalparameters that influence the interaction of the particles (of thematerial to be deposited) with the plasma jet 204 and the substrate 206,and therefore, the resultant deposit properties. These parametersinclude, by way of example and not by way of limitation, feedstock type,plasma gas composition and flow rate, energy input, torch offsetdistance, substrate cooling, among other possible parameters.

The resultant thin film 218 is comprised of a multitude of overlying andneighboring pancake-like lamellae also referred to as ‘splats’, formedas a result of the flattening of the liquid droplets 208 upon or afterstriking the surface of the substrate 206 (or previously depositedmaterial thereon). In embodiments in which feedstock powder is used, thelamellae may have thicknesses on the order of or smaller then the sizeof the feedstock powder particulates (or particles) but may have a muchlarger lateral dimension, perhaps 2, 10, 100, or more times thethickness. Between these lamellae, there are small grain boundariescomprising voids, such as pores, cracks and regions of incompletebonding. As a result of this unique lamellae-composite structure, thethin film 218 may have properties significantly different from thecorresponding bulk material properties. These may generally bemechanical properties, such as lower strength and modulus, higher straintolerance, and lower thermal conductivity. Also, due to the rapidsolidification, metastable phases can be present in the thin-filmdeposits.

The Plasma Spray Deposition techniques described herein in variousexample embodiments offer a number of extremely attractive attributes.By way of example, the use of large raw material feedstock batchesminimizes lot to lot variability. Furthermore, the feed source is immuneto process-induced modification. Additionally, Plasma Spray Depositionprovides extremely high throughput, good uniformity in the resultantthin film 218, large flat-grained structures, and broad flexibility inraw material selection, as well as tight control of stoichiometry in theresultant thin films. Furthermore, with the careful control of thedeposition rate and the molten droplet size the disclosed Plasma SprayDeposition techniques can be used on a variety of substrate materials.Thermal mismatch stresses can also be minimized by controlling theinterval between spray cycles as the film 218 is deposited.

Furthermore, in particular embodiments, photovoltaic absorber precursorthin films, or absorber thin films themselves, that have been depositedwith the Plasma Spray Deposition techniques described herein withreference to example embodiments can be subsequently processed toimprove their structural, chemical, and/or electronic properties. Inparticular embodiments, post processing annealing,hot-isostatic-pressing and/or other similar processes may be used toimprove densification and microstructural properties of the depositedfilms 218. These processes can be conducted in reactive environments toimprove the deposited film's structural, chemical or electronicproperties. By way of example, such subsequent processing may includeannealing at high temperature in the presence of sulfur (S), selenium(Se), or hydrogen sulfide (H₂S), among other suitable or desired gaseousenvironments. In particular embodiments in which it is desired tomaximize the chalcopyrite phase in a chalcogenide-based absorber layer,such post-processing may increase the conversion to the chalcopyritephases.

It should also be noted that the extremely large grains that are formedby the collision of the molten particles 208 with the substrate 206 havesizes (diameters) far in excess of what can be obtained by otherdeposition techniques where the grain diameters (the dimension parallelto the plane of the film) are typically on the order of the filmthickness (the dimension perpendicular to the plane of the film) (e.g.,on the order of 1 nm to 10 nm). However, in particular embodiments, thethin films deposited using Plasma Spray Deposition may exhibit anaverage grain size or average “grain diameter” of, for example, two,ten, a hundred or more times the thin-film thickness leading toidealized grain boundary minimization.

In addition, the feedstock materials that may be used to deposit thedesired thin film or films 218 can include the constituent elements,compounds, or alloys desired in the resultant deposited thin films insuitable combination. By way of example, for depositing CuInS₂ typeabsorbers for photovoltaic devices, the feedstock may be comprised ofCuInS₂ powder alone or in combination with one or more of CuGa, In₂S₃,and/or CuS powder, among others. In some embodiments, the specificfeedstock blend may also be altered during the Plasma Spray Depositionprocess to obtain compositional grading, if desired. By way of example,for the case of Cu(InGa)Se₂ (CIGS) absorbers the feedstock may compriseappropriate stoichiometric mixtures of Cu, CuGa, and/or In₂S₃ as well asvarious other elemental, alloy or compound mixtures. Indeed, anattractive attribute of utilizing Plasma Spray Deposition according toparticular example embodiments is the fact that additions can be made tothe feedstock to compensate for any stoichiometric variability betweenthe feedstock and deposited films 218. By way of example, additions maybe made to compensate for losses of more volatile or lower melting pointconstituents.

In particular embodiments, the feedstock can be compositionally variedwith or without the presence of additives (e.g. oxide particles) todeposit a variety of multilayer structured films or layers as well asmicrostructurally confined nanocrystalline semiconductor particles(commonly referred to as quantum dots). The resulting structures haveexcellent light absorption characteristics (high extinctioncoefficients) and are “tunable” so as to extend their wavelengthsensitivity and hence conversion efficiencies.

Furthermore, the present inventors have determined that Plasma SprayDeposition may improve the sticking of deposited layers 218 to eachother, to other deposited layers, or to the substrate 206. Moreparticularly, a conventional problem in the formation of photovoltaiccells is that the bottom contact layer (e.g., Molybdenum) tends not tostick to the underlying glass substrate and, in some instances, theadhesion between the absorber layer and the underlying bottom contactlayer is poor. However, the present inventors have found that plasmaspraying can improve the sticking. This may be a direct result of thelarger grains in the plasma sprayed layers. Neighboring grains may haveslightly different thicknesses at the boundaries. This mismatch mayactually improve sticking between adjacent layers (e.g., the bottomcontact layer and the glass substrate layer) through inter-penetratingsurface features; analogous to a mechanical stitching of the layers.

Again, it should be appreciated that the embodiments described hereinare not limited to the deposition of chalcopyrite photovoltaic absorbermaterials, but can be applied for all the other layers that constitute aphotovoltaic device such as, by way of example and not by way oflimitation, top and bottom contact layers, transparent conductivelayers, interlayers, and protective transparent layers. Moreover, plasmaspraying may be used for depositing silicon (Si) or other semiconductingcrystalline think films. By way of example, one could reuse or recyclethe Si powder leftover after slicing Si ingots into wafers or dicingwafers into dice to make Si thin films. One could also process the highpurity silicon powder produced from various fluidized bed processes.Plasma spray deposition may also be very advantageous in creatingmicrocrystalline silicon on glass type structures and providesubstantial film thicknesses without the attendant stress relatedfailures that are inherent to other deposition and fabrication pathways(for example, CVD, sputtering etc).

The present disclosure encompasses all changes, substitutions,variations, alterations, and modifications to the example embodimentsdescribed herein that a person having ordinary skill in the art wouldcomprehend. Similarly, where appropriate, the appended claims encompassall changes, substitutions, variations, alterations, and modificationsto the example embodiments described herein that a person havingordinary skill in the art would comprehend.

1. A method, comprising: providing a substrate suitable for use in aphotovoltaic device; plasma spraying one or more layers over thesubstrate, the average grain size of the grains in each of the one ormore layers being at least approximately two times greater than thethickness of the respective layer, wherein a grain size of a grain is ameasure of a dimension of the respective grain along a plane parallel tothe one or more layers and wherein the thickness of a respective layeris a measure of a dimension of the respective layer along a planeperpendicular to the one or more layers; and after one or more of theone or more layers are plasma-sprayed, subjecting the one or more of theone or more plasma-sprayed layers to one or more thermo-processes. 2.The method of claim 1, wherein the grain size of the grains in each ofthe one or more layers is at least approximately ten times greater thanthe thickness of the respective layer.
 3. The method of claim 1, whereinplasma spraying one or more layers comprises, for each layer to bedeposited over the substrate, injecting a feedstock material into aPlasma Spray Deposition system, and wherein the feedstock materialincludes the constituent elements, compounds, or alloys that form therespective layer upon plasma spraying the respective layer.
 4. Themethod of claim 3, further comprising altering the feedstock materialblend during the Plasma Spray Deposition process to obtain compositiongrading in the respective plasma-sprayed layer.
 5. The method of claim3, further comprising altering the feedstock material blend during thePlasma Spray Deposition process to compensate for stoichiometricvariability between the feedstock material and the respectiveplasma-sprayed layer including compensating for losses of relativelymore volatile or lower melting point constituents in the feedstockmaterial.
 6. The method of claim 5, further comprising adding one ormore additives to the feedstock material blend including one or morevarieties of oxide particles.
 7. The method of claim 1, wherein the oneor more layers comprise an absorber layer of the photovoltaic device. 8.The method of claim 7, wherein the absorber layer is comprised of achalcogenide material at least a substantial portion of which is in theform of a chalcopyrite phase.
 9. The method of claim 1, wherein the oneor more layers comprise a buffer layer of the photovoltaic device. 10.The method of claim 1, wherein the one or more layers comprise a topcontact layer or a bottom contact layer of the photovoltaic device. 11.The method of claim 1, wherein the one or more layers comprise atransparent conductive layer of the photovoltaic device.
 12. The methodof claim 1, wherein the one or more layers comprise a protectivetransparent layer of the photovoltaic device.
 13. The method of claim 1,wherein one or more of the thermo-processes comprise one or moreannealing or hot-isostatic-pressing.
 14. The method of claim 13, whereinone or more of the one or more annealing processes are carried out in ahigh temperature environment in the presence of one or more of sulfur(S), selenium (Se), or hydrogen sulfide (H₂S).